IstvanSz
Structural
- Dec 28, 2009
- 5
Hello All!
I am looking for some help in connection with the topic of floor response spectra.
I have a task that I need to generate a local response spectrum from an available floor spectrum. I need to transpose the floor spectrum to the top of a steel structure where an equipment will be located. I have limited tools and, well, my understanding of the topic is not very broad either.
I would like to ask you to give advice if the method I devised is good or if it needs improvement at some points.
The general idea is to generate a transfer function, T(f), so that I can get the local response spectrum as LRS(f) = T(f) * FRS(f), where f is frequency, LRS is the local response spectrum and FRS is the floor response spectrum.
I have an FE software that can calculate free vibration, can apply response spectrum as loads and can run a dynamic analysis with acceleration time history loading.
I am planning to take the following steps:
1) Run a free vibration analysis
2) Generate several single frequency harmonic loadings as time history loads (i.e. a sine wave):
- in the set of frequencies where the sine wave is generated I would include several frequencies near the resulting Eigen frequencies of step 1), and include few in between the result of step 1)
- the length of the load would be 10-20 cycles
3) For a given frequency calculate a point of T(f) as T(f) = a.max.result / a.max.timehist, where a.max.result is the maximum measured acceleration of a selected point on top of the steel structure, a.max.timehist is the maximum acceleration in the time history loading.
If I am not able to get the maximum acceleration as an output from the software then I will calculate the point of the transfer function as T(f) = f^2 * d.max.result / d.max.timehist, where d is the displacement.
4) repeat step 2) and 3) in the other 2 directions also
There are a few things that I am not fully sure of, and I am asking for your advice in connection with:
- Is the mass of the equipment needed to be included in the FE model in step 1) ?
- Is the mass of the equipment needed to be included in the FE model in step 2) ?
- What is the ideal length of the loading sine wave? Would it be best to seek some kind of convergence of the a.max.result?
I guess it is a natural question why I don't generate floor spectrum compatible artificial time history loading instead of the method described above. No doubt, that would be shorter way, but I dont know how to do that. If you can help me with a step-by-step guide in that topic, I would really appreciate that.
Regards,
Istvan
I am looking for some help in connection with the topic of floor response spectra.
I have a task that I need to generate a local response spectrum from an available floor spectrum. I need to transpose the floor spectrum to the top of a steel structure where an equipment will be located. I have limited tools and, well, my understanding of the topic is not very broad either.
I would like to ask you to give advice if the method I devised is good or if it needs improvement at some points.
The general idea is to generate a transfer function, T(f), so that I can get the local response spectrum as LRS(f) = T(f) * FRS(f), where f is frequency, LRS is the local response spectrum and FRS is the floor response spectrum.
I have an FE software that can calculate free vibration, can apply response spectrum as loads and can run a dynamic analysis with acceleration time history loading.
I am planning to take the following steps:
1) Run a free vibration analysis
2) Generate several single frequency harmonic loadings as time history loads (i.e. a sine wave):
- in the set of frequencies where the sine wave is generated I would include several frequencies near the resulting Eigen frequencies of step 1), and include few in between the result of step 1)
- the length of the load would be 10-20 cycles
3) For a given frequency calculate a point of T(f) as T(f) = a.max.result / a.max.timehist, where a.max.result is the maximum measured acceleration of a selected point on top of the steel structure, a.max.timehist is the maximum acceleration in the time history loading.
If I am not able to get the maximum acceleration as an output from the software then I will calculate the point of the transfer function as T(f) = f^2 * d.max.result / d.max.timehist, where d is the displacement.
4) repeat step 2) and 3) in the other 2 directions also
There are a few things that I am not fully sure of, and I am asking for your advice in connection with:
- Is the mass of the equipment needed to be included in the FE model in step 1) ?
- Is the mass of the equipment needed to be included in the FE model in step 2) ?
- What is the ideal length of the loading sine wave? Would it be best to seek some kind of convergence of the a.max.result?
I guess it is a natural question why I don't generate floor spectrum compatible artificial time history loading instead of the method described above. No doubt, that would be shorter way, but I dont know how to do that. If you can help me with a step-by-step guide in that topic, I would really appreciate that.
Regards,
Istvan
